When we think about “existential” threats, things that could potentially end the lives of everyone on Earth, most of the possibilities come from right here on our own planet—climate change, global pandemics and atomic warfare. Turning a paranoid gaze to the skies, we typically worry about asteroid strikes or perhaps some perilously massive burp from our sun.
But if you trust everything you read on the fringe regions of the internet, you may think the most fearsome heavenly threat may not only be extraterrestrial, but also extrasolar. Some 7,500 light-years away in the constellation of Carina a star called Eta Carinae, at least a hundred times more massive than our own sun, is approaching the point where it will detonate as a supernova. Simply put, Eta Carinae is a supermassive stellar powder keg nearing the end of its fuse. It could, in fact, already have met its doom, and the light bearing news of its cataclysmic death could be streaming toward us even now. Whenever that luminal death rattle arrives, tomorrow or tens of thousands of years in the future, there are two general sets of opinions about what would happen next.
The first opinion, held by various online alarmists I shall not indulge by linking here, holds that there would be a global mass extinction. This idea plays on fears that Eta Carinae’s supernova could unleash a gamma-ray burst (GRB), one of the brightest explosions in the universe. When a very massive star dies in a supernova, its core collapses in on itself, typically forming a stellar remnant, a neutron star or a black hole. If the core is spinning very fast, the stellar remnant will be spinning even faster, whipping a disk of material around its edges at nearly light-speed. Through processes still not fully understood this superheated and magnetized whirling disk then forms a pair of jets, like lighthouse beams, that blast out from its poles at relativistic speeds. The highly focused, extremely energetic emission from those jets is what we see as a GRB.
Over the years GRBs have been proposed as one of the reasons why we seem to be so alone in the universe—sooner or later, the thinking goes, most any inhabited planet will be struck by a GRB, blasting any biosphere practically to oblivion. And some researchers have speculated that one might have already hit Earth, at the end of the Ordovician period nearly 450 million years ago. Whatever did happen way back then, it managed to exterminate more than an estimated 80 percent of all species living at the time. It could be that even more GRBs hit our planet far earlier in its life, stifling the emergence of Earth’s biosphere until their cosmic prevalence fell below some critical threshold.
According to a somewhat plausible worst-case scenario, a direct hit by an extremely bright GRB generated by Eta Carinae could devastate our planet in a manner similar to but far worse than full-scale thermonuclear war. For several searing seconds, the planetary hemisphere facing the faraway star would be bathed in intense high-frequency radiation. The skies would fill with light much brighter than the sun, bright enough to ignite enormous continent-scouring wildfires on half the globe. The energetic burst of light would kick off atmospheric showers of highly penetrating radioactive subatomic particles called muons, which would stream down to poison life on the surface as well as that some distance underground and underwater. Even the far side of the planet facing away from Eta Carinae would not be spared, as the GRB’s intense energy would destroy the entire ozone layer while also sending superstorms rippling around the world. In the aftermath blackened, soot-filled skies would unleash torrents of acid rain, clearing only to soak the surface with damaging ultraviolet radiation. In a literal flash the Earth would become a planetary charnel house, and the shattered biosphere would require millions of years to piece itself back together.
The second opinion, held by most astrophysicists, is that Eta Carinae won’t produce a GRB at all—and if it did, it wouldn’t hit Earth. And even in a scenario where our planet did find itself in the crosshairs of a GRB from Eta Carinae, if the burst was of average brightness, its light would be too attenuated across 7,500 light-years to seriously harm the biosphere. In this scenario Eta Carinae’s demise would manifest as scarcely more than the star brightening to approach the luminosity of the full moon before gradually fading in the sky.
To understand how this stark divergence in opinion can exist it helps to know a bit more about Eta Carinae. Since first being catalogued by Edmond Halley in 1677, the star has fluctuated wildly in brightness, peaking in 1843 to become the second-brightest star in the sky for some two decades. Astronomers now consider that event a “supernova impostor”—instead of blowing apart the star ejected perhaps 10 percent of its total mass as two huge clouds of gas and dust, which is now known as the Homunculus Nebula. Glowing remnants of even earlier near-death experiences still wreathe the star. Viewed through a large telescope today, the total effect makes Eta Carinae look a bit like a peanut roasting in a fire.
Eta Carinae is shining so brightly that it is eroding itself, generating outward radiation pressure so intense that it almost counteracts the inward pull of gravity, sending its outer layers slowly streaming away on powerful stellar winds. Deep inside the star, below a thick outer envelope of hydrogen, fusion reactions are “burning” a variety of nuclear fuels in layers akin to those inside an onion. Eta Carinae’s past outbursts and pulsations are probably linked to instabilities between its inner layers created when it exhausted one nuclear fuel and transitioned to another.
Alex Filippenko, an astrophysicist at the University of California, Berkeley, says Eta Carinae’s massive envelope of hydrogen and strong stellar winds both reduce the likelihood of the star producing a GRB. “A thick hydrogen shell makes it difficult for a relativistic jet to pummel its way out of the star,” Filippenko says. “But if Eta Carinae doesn’t explode until quite a long time from now, there would be enough time to get rid of the outer shell, and it would then be more likely to become a GRB.” Except, he adds, once the outer shell is gone, the stellar winds would likely increase in strength, dissipating much of the angular momentum that would be required to spin up a GRB when Eta Carinae’s core collapses. “All this makes a GRB less likely, but not impossible,” Filippenko says. “And even if it gets rid of its hydrogen shell prior to exploding and does become a GRB, [Eta Carinae] is probably not pointing at us right now.” The twin lobes of Eta Carinae’s Homunculus Nebula are tilted away from us at an angle of about 40 degrees whereas Filippenko says a GRB emerging from a collapsing star’s polar axis would have a spread of about 10 degrees or less. So if the Homunculus Nebula is aligned with Eta Carinae’s polar axis, an emitted GRB would miss our solar system by a very wide margin.
Unfortunately, there is one major complication to this picture: Astronomers discovered in 2005 that Eta Carinae is actually a binary system with a relatively small companion of “only” 30 times the mass of our sun in an approximately five-year orbit around the 100-solar-mass star. If the smaller companion doesn’t orbit in alignment with the more massive star’s rotational axis, then the Homunculus Nebula might not be aligned with the massive star’s poles. And, conceivably, gravitational interactions between the two stars, or with another passing star, could shift the orientation of the more massive star’s axis, potentially aiming it right at us. Finally, the presence of the companion star could also alter how the more massive star evolves, throwing more uncertainty into the timing and mechanics of any eventual supernova.
Piled one atop the other, all those variables are in large part why Eta Carinae is “our biggest embarrassment today,” says Stan Woosley, an astrophysicist at the University of California, Santa Cruz, who specializes in modeling the evolution and death of stars. “No one knows just what’s going on there…It could die tomorrow or a long time from now.”
Some of what happens next depends on which nuclear fuel is currently dominant inside Eta Carinae. If it is fusing elements such as oxygen or carbon in or near its core, it may only have years to live, centuries at most, and could soon eject its outer envelope of hydrogen. If its core is instead fusing helium, the star could potentially shine on for a few hundred thousand years more. Alternatively, helium fusion could cause Eta Carinae to swell up like a balloon to become a supergiant star, in which case its smaller companion star might enter and disrupt the outer hydrogen envelope, hastening the supergiant’s explosive death.
Once the star dies, Woosley says, its core will likely collapse to form a black hole, although one rotating too slowly to make a relativistic disk and a GRB. Without the creation of such a disk the death of Eta Carinae could be “particularly unspectacular,” failing to even produce a supernova as the star’s remnants simply slip behind the black hole’s event horizon.
“Sometimes I wonder if Eta Carinae already has,” Woosley says. “But people tell me they can still see the star.”